The present invention relates to a speed controlling apparatus for a brushless motor.
In recent years, so-called brushless motors which have electronic switches using transistors, instead of the conventional DC motor provided with a mechanical switch mechanism such as a brush or commutator, are often used as driving motors for acoustic appliances and image appliances in order to have longer service life, higher reliability or a thinner shape. Also, although the counter-electromotive voltage caused in the driving coils is used or the AC tachogenerator for practical use as the speed controlling apparatus for the brushless motor is used, the use of the counter-electromotive voltage is generally advantageous in terms of construction simplicity, economy, etc. For example, "Base and Application of Precise Small Motor", written by Hiroshi Yamada (published by Sogo Denshi Shuppansha Co., Ltd., of Japan, July 1, 1975) shows the conventional art of the controlling apparatus using the counter-electromotive voltage at page 234.
One example of the above-described conventional motor speed controlling apparatus will be described hereinafter with reference to the drawings.
FIG. 5 is a circuit connection diagram for the motor speed controlling apparatus, which includes the positive-side and negative-side feeder lines 1 and 2 of the power supply and motor driving coils 3 through 6. One of each respective end of the driving coils 3 through 6 are connected with the negative-side feeder line 2. The respective ends are connected with the collectors of the driving transistors 7 through 10 and are connected with the cathode side of the diodes 11 through 14. The anode side of the diodes 11 through 14 are connected in common and are connected with the emitters of the transistor 15. The hall elements 16 and 17 are adapted to detect the positions of the permanent magnet rotors (not shown). One output terminal of the hall element 16 is connected with the base of the driving transistor 7, the other output terminal which is adapted to output a signal which has a phase difference of 180 degrees from the output terminal and is connected with the base of the driving transistor 8. The mutual emitters of the driving transmitters 7 and 8 are connected in common with the positive side feeder line 1 through the resistor 18. One output terminal of the hall element 17 is connected with the base of the driving transistor 9, the other output terminal which is adapted to output a signal which has a phase difference of 180 degrees from the output terminal and is connected with the base of the driving transistor 10. The mutual emitters of the driving transistors 9 and 10 are connected in common with the positive-side feeder line 1 through the resistor 19. One of each respective terminal of the hall elements 16 and 17 are respectively connected with the positive-side feeder line 1, also the other terminals are respectively connected with the collector of the transistor 22 through the resistors 20 and 21. The emitter of the transistor 22 is connected with the negative-side feeder line 2, and the base is being connected with the positive-side feeder line 1 through the resistor 26, with the collector of the transistor 15. The series circuit of the capacitor 23, the resistor 24 and the capacitor 25 are connected between the the collector and the base of the transistor 22. The base of the transistor 15 is connected with the positive-side feeder line 1 through the resistor 27, with the negative-side feeder line 2 through the series circuit of the variable resistor 31 and the resistor 32, with the emitter through the series circuit of the resistor 28 and the resistor 29. A thermistor 30 for temperature compensation is connected with both the ends of resistor 29. Also, the capacitor 33 is connected between the base and the emitter of the transistor 15.
The operation will be described hereinafter with reference to the motor speed controlling apparatus constructed hereinabove.
The motor shown in FIG. 5 is a brushless motor of four-phase one-direction energization, using two hall elements 16 and 17 and the four driving coils 3 through 6 as described hereinabove. Namely, the current controlled by the hall elements 16 and 17 through the permanent magnet rotator sequentially flows into the driving coils 3 through 6 which are positioned every 90 degrees so as to cause the rotating magnetic field. In this case, the current flowing into the driving coils 3 through 6 is proportional to the current flowing into the hall elements 16 and 17. The current flowing into the hall elements is turned into a value proportional to the speed error to control the motor torque.
Namely, the driving transistors 7 through 10 are sequentially switched. During the energization period that exists when the current does not flow into the driving coils 3 through 6 caused by the switching operation, the counter-electromotive voltage to be caused in the driving coils 3 through 6 is rectified by the diodes 11 through 14, smoothed, compared with the reference voltage and simplified in difference. The current proportional to the speed error flows into the transistor 22 controlling the current flow into the hall elements 16, 17 which controls the feed power amount of the driving transistors 7 through 10 so as to render the rotation speed constant.
As the rectification is effected across the diode to detect the counter-electromotive voltage in such construction as described hereinabove, the sequential direction voltage is changed by ambient temperature, This causes an error in the detection voltage of the counter-electromotive voltage, so that the rotation speed is changed. Thus, a thermistor 30 for temperature compensation or the like is required as in the conventional example of FIG. 5. Also, it is impossible to detect when the value of the counter-electromotive voltage caused is less than or equal to the sequential direction voltage of the diode.
Also, as the resistors 18, 19 are connected with the emitters of the driving transistors 7 through 10 in the above-described construction, the maximum voltage which may be applied across the driving coils 3 through 6 is reduced through the voltage drop. Namely, there is a problem that the controllable maximum torque of the motor and the starting torque will be lowered. Thus, in a motor where no tolerance of torque with respect to the power-supply voltage is allowed, the insertion of the resistors 18 and 19 is impossible to be performed. Accordingly, it is extremely difficult to produce uneven control, or uneven torque, which will be caused by the characteristic dispersion of the driving transistors 7 through 10. Also, the resistors 18, and 19 often require high power consumption and are disadvantageous despite in terms of lower prices, and smaller space.